"He's incredibly quick-witted. Everything he brings to Tony Stark is him. Of course there's all of the tradition from the comic books, the movie, and also the writing is brilliant, but there's also a heavy dose of that character is Robert Downey Jr. "

The Parade's End actress added: "He's inspiring to work with and watch. He's a force."

Guy Pearce and Sir Ben Kingsley are also set to star in the film, which opens in April, alongside Gwyneth Paltrow, who returns as Pepper Potts.